The invention relates to a controlled grafting process of a polyolefin in the presence of radical reaction initiators.
Radical grafting of polyolefins is a method widely used in the industrial field for the production of modified materials, essential in many plastics formulations and used, for example, as chemical coupling agents, impact modifiers or compatibility enhancers for fillers. The grafting reaction allows the introduction into the polymeric chain of small quantities of polar groups, for example of anhydride or acid ester nature (contained in general in small percentages by weight), with the object of imparting new properties to the polymer without significantly varying its starting characteristics. Usually, the reactive transformation process is conducted in discontinuous mechanical mixers or in extruders, at a temperature which makes it possible to maintain the reaction mixture in the molten state.
The reactive mixture also comprises, beyond the polyolefin, at least one radical reaction initiator and at least one grafting compound. Usually, the initiator is a peroxide, whilst the grafting compound, possibly in mixture with other compounds, is an unsaturated polar compound, such as, for example, maleic anhydride, a maleic ester, a maleic semiester or a methacrylate.
In the reaction mechanism the radical reaction initiator, a peroxide, decomposes into peroxide radicals (RO′) at high temperatures. The peroxide radicals can be stabilised with termination reactions, combined with a grafting compound (M) or extract a hydrogen from the polyolefin (P) by generating macroradicals (P.). The macroradicals (P.) of the polyolefin can give cross linking products, degradation products or interact with grafting compounds to give other radicals (PM.). The grafted polyolefins (PMH) are generated when the macroradicals (PM.) of the grafted polyolefin become stable by extracting a hydrogen from another molecule of polyolefin, which in turn becomes radical (P.).
However, because of the great reactivity and consequent low selectivity of the free radicals in their reactive activity, the grafting of the polyolefin occurring in the melt by radical means is accompanied by collateral reactions which can be attributed to the degradation reactions and to cross linking reactions of the polyolefin. When these reactions occur, which compete with the grafting, the process therefore has a reduced overall efficiency and a final product with a low degree of grafting. Another consequence of the degradatory and cross linking reactions is the variation in the average molecular weight of the polyolefins.
In the treatment of polypropylene with peroxides, for example, the phenomenon of rupture of the macromolecular chain through β-splitting reactions is predominant. The macroradical coupling reactions which lead to the formation of cross linked polymers are, in fact, notably slower than β-splitting because of both the ratio between the velocity constants of the two processes, which promotes degradation, and of the biomolecularity of the kinetics of the coupling process. The overall effect of the addition of a peroxide to polypropylene therefore leads to a diminution of the average molecular weight and an increase in the “melt flow rate” (MFR) with strong variations in the primary structure of the polymer.
The association of maleic anhydride (MAH) in the treatment of the polypropylene with peroxides to obtain grafting of the polymer leads to similar results. In fact, a drastic reduction in the molecular weight is evident for any grafting compound/initiator ratio and a dependence of the degree of final grafting on this ratio is evident. It is known, moreover, that the coupling of the MAH takes place principally on the primary radical deriving from the β-splitting reactions.
By working in the melt, therefore, it is rather difficult to obtain a good compromise between the radical grafting reactions and the β-splitting reaction of the polymeric chain. The problem is essentially tied to the formation of unstable tertiary macroradicals which give rise preferentially to β-splitting reactions.
To limit the degradatory effect and, therefore, the β-splitting reactions, radical grafting procedures are known (referred in particular to the grafting of polypropylene) which utilise molecules or systems of molecules to put along-side known grafting compounds and which are able to convert the macroradicals into radicals less subject to β-splitting reactions.
One example is the radical grafting assisted by styrene (STY). In the grafting of polypropylene (PP) with glycidyl methacrolate (GMA), slightly reactive to macroradicals of the polymer, the styrene can be utilised as a grafting co-agent. The styrene reacts in the first place with the macroradicals of polypropylene giving rise to radicals of the more stable styrilic type less subject to the degradation reaction. Subsequently these radicals co-polymerise with GMA. In this way, instead of directly grafting the GMA on the polymeric chain, the STY is inserted as a bridge between the PP and the grafting compound. It is deemed, moreover, that GMA reacts more easily with the styrilic radicals, than with macroradicals of PP: consequently, a synergic effect takes place which further limits the degradation of the polymer.
Similar results are obtained although with different reaction mechanisms by utilising styrene in the radical grafting of polypropylene with maleic anhydride.
The use of the styrene as grafting co-agent is limited, however, by the fact that only some grafting compounds succeed in effectively copolymerising with the styrilic macroradicals with effective inhibition of the degradatory processes. The procedure is only useable, however, to insert some functional groups into the polyolefin chain, limiting the possible applications of the final grafted products.